The present invention relates to an apparatus for controlling the drum water level in a drum boiler. More particularly, the invention pertains to a drum water level controlling apparatus which may be suitably employed to suppress a variation in the drum water level which occurs when the load changes rapidly and by a large margin, for example, at the time of load run back.
Drum boilers are widely employed as boilers for thermal power generation. The water/steam system in a thermal power plant which employs a drum boiler is generally arranged as follows. In this type of boiler, water is fed into a drum by a feed water pump. The drum is further supplied with water and steam from a lower header through a riser tube which also serves as a water-wall tube. In the drum, water and steam are separated from each other, and the separated water is fed to the lower header through a downcomer. The steam which is generated in the drum is passed through a 1st superheater, an attemperator, a 2nd superheater and a turbine control valve and is then applied to the turbine so as to drive the same. The steam which has been used to drive the turbine is sent to a condenser where it is condensed into water. This water is supplied to the drum again through the above-described feed water pump. It is to be noted that the attemperator regulates the main steam temperature, that is, the temperature of the steam introduced into the turbine, by spraying the steam with a part of the water introduced thereto from the feed water pump.
It is important for this drum type boiler to control the water level within the drum. If the water level falls, the inside of the drum may be overheated, and this unfavorably causes the metal portion of the drum to melt and an abnormal pressure to be generated. A rise in the water level causes steam which has not been superheated to be introduced into the turbine, in which case, the metal portion of the turbine may be corroded. For this reason, when the water level abnormally varies in either case, the boiler system is tripped off. For the same reason, the water level control in a normal or ordinary state is effected within the range between limited water levels. More specifically, as also disclosed in the specification of Japanese Patent Laid-Open No. 87703/1981, the flow rate of feed water is controlled in accordance with a signal obtained by correcting the deviation of a real water level from a reference value for the water level by the main steam flow rate or the feed water flow rate. In this case, the main steam flow rate and the feed water flow rate act as anticipatory control signals so as to quicken the response time of the water level control system.
When an important auxiliary machine in the plant is out of order, load runback is carried out in which the load is rapidly and greatly reduced to the level at which the remaining auxiliary machines can continue the operation. However, in such load runback, the water level in the drum undesirably varies by a large margin, so that the water level may disadvantageously exceed the upper or lower limit value to reach a boiler trip level. This problem will be explained hereinunder in more detail.
When load runback is started, as is well known to those who are skilled in the art, the water level in the drum temporarily lowers and, therefore, the feed water flow rate is controlled such as to be increased through the proportional plus integral action. However, if the increase in the feed water flow rate is not adequate, the water level in the drum may fall to an abnormally low level, to the lower limit value to reach the boiler trip level (low trip level). It is considered that the above-described temporary lowering of the water level is caused by the fact that the pressure inside the drum is raised by the rapid throttling down of the turbine control valve which is effected by the load runback, thereby causing air bubbles contained in the water in the drum, the downcomer, the lower header and the riser tube to be compacted.
Even if the water level in the drum is saved from reaching the low trip level due to an increase in the feed water flow rate, the run down of the feed water flow rate effected by the proportional plus integral control cannot cope with a sudden rise in the drum water level after the load runback, so that there are many occasions where the water level undesirably rises in excess of the upper limit value to reach a boiler trip level (high trip level).
The load runback carried out in relation to the drum type boiler will be successful only when it overcomes both the rise and fall in the water level in the boiler.